From the Super Mario Wiki
An Invincible Urchin is an item in Mario Kart Arcade GP DX. It is a blue ball with spikes surrounding it, resembling a Spiked Ball. When used, three Invincible Urchins surround the user similar to a Triple Green Shell causing other racers to spin out if they touch the Urchins. Unlike Triple Green Shells, they do not disappear when this happens, although they automatically disappear after a couple seconds, similar to a Super Star.